Flame Test (Positive ions)
- Put a small amount of the compound to be tested in a nichrome loop.
- Then hold the loop in a blue flame of a bunsen burner.
- Then use the colour of the flame to identify the metal ion.
Lithium (Li+) = Crimson
Sodium (Na+) = Yellow
Potassium (K+) = Lilac
Calcium (Ca2+) = Red
Barium (Ba2+) = Green
Reactions with Sodium Hydroxide
- Aluminium, Calcium and Magnesium ions form white precipates, so when we react it with sodium hydroxide we know that the unknown compound cantains either; Al3+, Ca2+ or Mg2+ ions.
- If we add more sodium hydroxide then the white precipitate formed with aluminium ions dissolve.
- Calcium and Magnesium ions can be distinguished by a flame test. Magnesium ions produce no colour.
We can identify ions in a substances by following this:
Copper ions = Blue precipitate
Iron ions (l) = Green precipitate
Iron ions (ll) = Brown precipitate
Carbonates (negative ions)
- If we add dilute acid to a cabonate it fizzes and produce carbon dioxide.
- This is a good test to see if an unknon substance is a carbonate.
- In limewater the carbon dioxide reacts with calcium hyroxide. It forms a white precipitate of calcium carbonate which turns the limewater cloudy.
Halides (chloride, bromide and iodide)
- The test shows whether chloride, bromide or iodide are present in the compound.
- First we add dilute nitric acid, then add silver nirtrate solution, this removes any carbonate ions.
- If a precipate forms then a halide is present in the compound.
To indentify what ion is present we need to look at the colour of the precipitate:
Chloride ions = White precipitate
Bromide ions = Cream precipitate
Iodide ions = Pale yellow precipitate
Sulfates (negative ions)
- We can test for sulfate ions by adding dilute hydrochloric acid, followed by barium chloride solution.
- The dilute hydrochloric acid removes carbonate ions .
- A white precipitate tells us sulfate ions are present